This invention relates generally to coil boilers of the flash type, and more particularly to those where the coil arrangement is "straight through", wherein all heated coils are arranged to introduce feedwater at one end and exit steam and water at the other. This type of boiler is highly responsive to demand for steam, and quickly responds to varying steam flow demands as well as changes in required steam pressure. Rapid response is due to the relatively small amount of water undergoing heating within the coils at any given time, a feature which, in addition to the previously indicated advantages, provides a relatively safe boiler since high pressure steam and/or water vapor exists in only a small portion of the heated coils.
However, the highly responsive nature of these boilers requires extremely close control of the coil feedwater input. Close feedwater control is required due to the extremely small amounts of water undergoing the transition from liquid to vapor state in producing steam, and the relatively small mass of the heated coils. In addition, the boiling internal of the coils can involve essentially all of the known boiling modes, i.e. incipient, nucleate, and transient film boiling.
The need for close feedwater control is further exacerbated by applications of these units wherein rapid changes in demand and/or required pressure are commonly encountered. Under these conditions, if feedwater inputs to the coil are not precisely matched to the output for demand, coil damage due to either lack of a continuum of flow and maintenance of proper control of dissolved feedwater solids, or overheating due to complete absence of water will occur. In any case, it is necessary to maintain certain minimum flows at maximum output pressures and prevent excessive boiler feedwater flow at reduced demands and pressures, or any combination of these operating conditions.
Prior approaches of feedwater control in flash boilers are disclosed in U.S. Pat. Nos. 2,735,410, 3,425,622, and 3,532,028, each hereby fully incorporated by reference. These approaches, while providing reasonably acceptable boiler operation, suffer from the common disadvantage of relatively narrow operating ranges of boiler pressure and steam flow or output. Although these units compensate for mass flow variations by decreased firing as feedwater temperature increases, compensation for feedwater enthalpy is not provided. Boiler operation over a usually encountered feedwater temperature increase is therefore unsatisfactory, due to generating superheated steam.
This difficulty requires that each boiler or steam generator application be equipped with a "tailored" controller flow characteristic, in order to prevent the above mentioned difficulties.
It is therefore an object of this invention to overcome the deficiencies of existing feedwater controllers by providing a feedwater/combustion controller having a wide dynamic range of boiler operating temperatures, pressures, and flow rates.
It is a further object of this invention to provide a feedwater controller for a flash type steam generator wherein compensation for the variations in enthalpy of delivered steam and/or feedwater is provided.
It is an additional object of this invention to provide a feedwater controller for a flash type coil steam generator wherein minimum and maximum feedwater flows over a wide range of operating pressures and temperatures, provides increased coil life through maintenance of minimum feedwater flows and prevention of coil burnout.